Apparatus for pulling single crystals in the form of long flat strips from a melt



I July 1963 G. K. GAULE ETAL 3,096,158

APPARATUS FOR PULLING SINGLE CRYSTALS IN THE FORM OF LONG FLAT STRIPS FROM A MELT Filed Sept. 25, 1959 3 Sheets-Sheet 1 F/G/ F/GZ v INVENTORS, f3 GERHART K. GAULE HORST H KEDESDY LEON J. D. ROUGE.

E WZ QJQm mg July 2, 1963 G. K. GAuL ETAL 3,

APPARATUS FOR PULLING SINGLE CRYSTALS IN THE FORM OF LONG FLAT STRIPS FROM A MELT 3 Sheets-Sheet 2 Filed Sept. 25, 1959 lNVENTORS, GE/PHART K. GAULE HORST h. KEDES'DY LEON J. D. ROUGE. BY

@MQM

ATTORNEY July 2, 1963 G K. GAuLE ETAL 3,

APPARATUS FOR PULLING SINGLE CRYSTALS IN THE FORM OF LONG FLAT STRIPS FROM A MELT Filed Sept. 25, 1959 3 Sheets-Sheet 3 INVENTORS,

GERHART K. sAuL HORST H. KEDESDY LEON J. o. ROUGE BY dmaw rg ATTOR N EY.

United States Patent 3,096,158 APPARATUS FOR PULLING. SINGLE CRYSTALS IN I'I IHE FORM OF LONG FLAT STRIPS FROM A ELT Gerhart K. Gaul, Long Branch, Horst H. Kedesdy, Little Silver, and Leon J. D. Rouge, Fort Monmouth, NJ., assignors to the United States of America as represented by the Secretary of the Army Filed Sept. 25, 1959, Ser. No. 342,536 2 Claims. (Cl. 23-473) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

The present invention relates to the art of growing crystals, and more particularly to methods and apparatus for growing thin single crystal strips from a melt.

A great need exists in the design and production of various semi-conductor devices for flat single crystal strips of germanium or silicon having a uniform rectangular cross-section. The single crystal material is usually grown from the melt in the form of rods having a few centimeters in diameter and up to 20 cm. long. To produce the strips from such a rod, several sawing, grinding and etching processes are necessary which entail much waste of precious material, :delicate and expensive tools for handling and testing. Attempts have been previously made to manufacture the strips by more direct methods such as by rolling, forging, or casting.

The strips produced by such mechanical processes were either not single crystals, or else the ensuing damage to the crystal structure and the amount of contamination were intolerably high.

Another method described in the prior art for pulling single crystals of germanium consists in placing a flat seed crystal in contact with a melt and pulling it through a restricted aperture of an electrically heated graphite cover. Among other disadvantages, this method is subject to the serious double limitation in that the crosssection of the growing crystalline ingot cannot be maintained uniform, and the method is applicable to only few semiconductors.

The prior art processes share the common greatly undesirable characteristic of producing ingots of variable cross section. If an attempt is made to physically control the cross section by pulling the growing crystal through a restricted aperture, the resulting contamination renders the crystal useless for most purposes. An additional disadvantage may ensue in that the passage of the crystallizing ingot of some semiconductor materials through a constrained aperture, if made in an electrically conductive material, may produce hazardous environmental conditions.

It is therefore the object of the present invention to avoid the undesirable shortcomings of the prior art by providing a simple method and relatively inexpensive apparatus for producing uniformly flat strips of single crystalline structure without contamination, and thereby eliminate the great waste inherent in the mechanical processes for handling either fiat or round ingots of non-uniform cross section. V

The essence of this invention resides in the discovery of indirect means for controlling the shape of the forming crystal without affecting the single-crystal structure thereof and without introducing any contamination therein. The physical pressure required to control the width or the thickness of the growing flat crystal is obtained electro-magnetically from a specially designed radio-frequency induction coil. The geometrical shape of the coil and the frequency of the R-F current circulating therein determines the direction and magnitude of the applied pressure on the flat surfaces of the forming crystal.

The nature of this invention will be more fully understood from the following detailed description and by reference to the accompanying drawings in which:

FIGURE 1 is a perspective view of a growing flat crystal when pulled in the vertical direction from the liquid melt;

FIGURE 2 is a top View of the crystal of FIGURE 1;

FIGURE 3 is a sectional view of the crystal of FIG- URE 1 along the X-X axis;

FIGURE 4 is a sectional view of the crystal of FIG- URE 1 along the Y Y axis;

FIGURE 5 shows the application of pressure to the crystal of FIGURE 4 for controlling the thickness there of in accordance with this invention;

FIGURES 6a and 612 show a radio frequency induction coil for exerting the forming pressure in FIGURE 5;

FIGURE 7 shows the effect of the geometrical shape of the induction coil on the formation of the crystal;

FIGURES 8a and 8b are elevational views partly in section of an apparatus of one embodiment of this invention for pulling a crystal from the melt;

. FIGURES 9a and 9b are elevational views partly in section of another embodiment for pulling the crystal from a solid semiconductor ingot;

FIGURE 10 is yet another embodiment for pulling wide flat crystals from previously prepared strips.

FIGURE 11 shows in cross-section a radio-frequency Eoil employing a field concentrator and an energy reector;

FIGURE 12 is a top View of the concentrator and reflector; and

FIGURE 13 is a diagrammatic view of the field concentrator showing its effect on the interface region of the growing crystal.

In FIGURES 1 and 2, the growing solid crystal 1 is pulled vertically from theliquid melt 2. Although the crystal may be pulled in any axis, usually some critical selection of axis is made depending on the working substance. In silicon and germanium, for example, the (112) axis maybe chosen as the axis along which the strip is grown, because the (111) planes are known to be especially stable during crystal growth and particularly suitable for manufacturing purposes. T he growth interface 3 is the plane of separation between the solid state and the liquid state. The lower part of the crystal adheres to the molten material. The free surfaces of the liquid and the solid meet on the boundary of the growth inter-face at different angles due to the cohesive forces between the molecular particles of the melt. Two reference angles will be presently defined.

In FIGURES 3 through 5, 0c is the angle delimited by a vertical plane parallel to the YY axis and a plane tangent to the surface of the free liquid at point 0 on the interface, and p is the angle delimited by a vertical plane parallel to the X-X axis and a plane tangent to the surface of the free liquid at point P on the interface. The vertical'height between the growth interface and the horizontal plane of the free liquid is denoted by the letter h.

The laws of surface tension which govern the shape of the free surface of a liquid indicate that, in a set-up as described above, 18 is always greater than a. This result has been experimentally verified. By appropriate choice of the height h of the interface over the bulk of the melt, or some other means, on can usually be adjusted to zero, and, when this is done, there will be no appreciable change in the width of the crystal during its growth. The angle 5, however, cannot be reduced to zero by conventional means and, consequently, the crystal layers exceeding the growth interface in the YY dimension will be successively incorporated into the growing crystal and result in a constantly increasing thickness dimension.

Thus, even though the width of a growing crystal can be fairly well maintained within predetermined limits, the thickness thereof, however, cannot be simultaneously delimited by any previously known means without destroying the single crystal structure, or without introducing an intolerable amount of contamination.

Hence, merely using a fiat seed crystal in a conventional pulling mechanism, without the application of indirect means for exerting physical pressure on a critical area of the growing crystal, will not produce strips of uniform cross-section.

In FIGURE 5, ,8 is reduced to zero by the application of physical pressure, indicate-d by the small arrows, in a direction perpendicular to the flat surfaces of the liquid material which is just below the growth interface. Making equal to zero produces a crystal of uniform thickness.

In FIGURE 6(a), the means in accordance with this invention for producing the physical pressure indicated in FIGURE 5, is an especially designed flat induction coil 4 surrounding the solid-liquid region, as can be clearly seen from FIGURE 6(1)). The coil is connected to a radio-frequency current generator, not shown.

When an R-F current flows in the fiat induction coil 4 the magnetic flux induced into the liquid-solid region of the crystal goes through variations in intensity and direction that corresponds to the alternations in the radiofrequency current. Variations in the magnetic field, in turn, generate eddy currents within the crystal according to well known elementary principles. The inter-action between the currents in the coil and the eddy currents produces physical forces the resultants of which are inwardly directed and perpendicular to the molten surfaces at the interface. The direction and magnitude of the resultant forces depend, among other things, on the location and geometrical configuration of the induction coil, and on the intensity and frequency of the applied current. With a properly designed flat coil the distributed force or pressure will, for the most part, be exerted against the flat sides of the liquid material, so that [i is reduced to zero. The repulsive forces generated by the interaction between the current in the coil and the eddy currents at the interface will increase the amount of pressure on that portion of the liquid which moves closer to the inner edge of the coil. Stating it differently, for a given current intensity, the pressure at any point on the interface is roughly inversely proportional to its distance from the inner edge of the coil. Hence, the pressure at any point may be expressed mathematically as a function of the radio-frequency current in the coil and its distance from the interface. The frequency of the applied current affects the penetration depth of the eddy currents and, therefore, their heating effect on the liquid solid region. For semiconductors such as germanium or silicon, it was found that the frequency should be greater than 100 k.c.; optimum performance was obtained with a frequency close to 0.5 megacycles.

In FIGURE 7 is illustrated the effect of the geometrical shape of coil 4 on the growing crystal 1. To obtain especially thin strips a coil 4' having a dumbbell-shaped cross-section is utilized. Since the physical pressure is greater at the central region, the cross-section of the growing crystal, 1' will assume the general shape of the cross-section of the coil as indicated at 3'. Thus, it is possible, by suitably shaping the coil to grow directly single crystals possessing various highly desirable geometrical configurations. The cross section of the coil acts as an indirect mold for the growing crystal. Since the mold is indirect, i.e., it is not in physical contact with the melt, it does not introduce any undesirable contaminations, nor does it destroy the single crystalline structure of the crystal.

In FIGURES 8a and 8b a simple apparatus utilizing a rectangularly shape-d compression induction coil 14 is shown. A flat seed crystal :12, produced by a previous operation or by conventional mechanical processes, is mounted on a cooled seed-holder 11 and is held in place by two screws 10. The seed is dipped slightly into the melt 15, then it is slowly withdrawn therefrom. The melt is placed in a suitable crucible '16 and heated by a conventional heater or by an ordinary induction coil 17.

A new crystal 13- starts to grow onto thelower end of the seed 12. It has a tendency to grow outwards, and, particularly, to spread in the thickness direction. The funnel-shaped rectangular compression induction coil 14 is so designed, in accordance with the previously discussed general principles, that its threefold function is: (l) to control the temperature at the surface of the melt 15 so that outward crystallization is limited to the crosssectional area of the seed crystal only, -(2) to raise the liquid-solid interface 18 (shown by dashed lines) of the growing fiat crystal slightly above the surface of the melt in the crucible, and (3) to apply physical pressure to the flat surfaces of the growing crystal so as to maintain the cross-sectional area of the growing crystal =13 uniform, i.e., to make the angle [3 of FIGURE 5 equal to zero.

In FIGS. 9a and 9b, the tip of a monocrystalline or polycrystalline rod 27 is melted by a circular induction heater coil 26. Above heater coil 26 is placed the compression coil 14 having a rectangular cross-section and a funnel-shape. A flat crystal seed 12 mounted in a cooled seed-holder 11 is lowered into the molten tip 25 and is slowly withdrawn therefrom in a vertical direction. The functions of the pressure coil 14 are the same as those described in conjunction with FIGURE 8. To continue the growth of the fiat crystal 13 rod 27 is moved slowly upward through the induction coil systems.

This method has an advantage over the method described in relation to FIGURE 8 in that the pressure coil 14 can be made to surround the molten floating zone 25 directly opposite the interface, as shown in cross section in FIGURE 6(b). This desirable physical location of the pressure coil allows greater control over the reduction of angle [3. In the present method, however, the width of the grown fiat single crystals is limited by the diameter of rod 27.

In FIGURE 10 is shown a method for growing wider flat single crystals than those obtainable by the previously described methods. Thin strips 30 are cut from a crystal ingot or polycrystalline rod and are mounted side by side in a strip holder 31. A rectangularly shaped pressureheater coil 34 melts the strips 30 at their tips. A wide flat seed crystal 12 mounted in a cooled seed-holder 11 is lowered into the melted tips 32 and is slowly withdrawn therefrom. A continuous flat crystal growth is obtained by moving the strip holder 31 slowly upward.

The single turn pressure-heater coil 34 of this embodiment is designed to perform the same functions as the pressure coil 14 and the heater coil 26 of the embodiment of FIGURE 9, i.e., it l) melts the tips 32 of strips 30, and (2) it exerts physical pressure on the liquid-solid interface region to maintain the cross section of the growing crystal uniformly constant.

In FIGURES 11 through 13 is shown a more desirable and more elficient design of the previously described pressure coil.

Two oval-shaped copper cones 40 and 41 are inserted inside the induction coil 43. Cone 41 has a narrow slot 42 extending from the rectangularly shaped aperture 48' to its upper edge. The last turn 43' of coil 43 is soldered to cone 41 at point 44 without shortcircuiting the cone at the slot. All other turns of coil 43 are electrically insulated from the body of cone 41.

Coil 43 and concentrator 41 act as an auto-transformer, as will be presently explained. Each turn of coil 43 induces electromagnetically currents I, in cone 41, three typical paths of which are shown in FIGURE 13. Cone 41 is made of a highly conductive material so as to provide a low resistance path for the electromagnetic energy generated by induction coil 43. Due to the existence of slot 42, the induced currents I, in cone 41 cannot flow in circles around the surface of the cone, but must follow paths as shown in FIGURE 13, i.e., all induced currents are turned by slot 42 towards aperture 43. The concentration of the induced currents around the edges of aperture 48 produces a very strong electromagnetic field in the aperture region. This field, in turn, induces strong eddy currents 1 at the interface 47 within the melt 49 for the growing crystal. The polarity of the induced currents I, and the eddy currents 1 are in opposite directions, as shown by the small arrows. The repulsive forces created by the oppositely circulating induced and eddy currents in the aperture region physically shape the geometrical cross-section of the growing crystal. The induced currents in the flat edges of the aperture closest to the interface produce very strong repulsive forces 66 and the currents in the rounded edges 48, being more remotely spaced from the interface, create only weak repulsive forces 61 and 62. Thus, the cross-section of the growing crystal can be controlled by the geometrical configuration of the aperture.

The eddy currents T generate heat within the interface region depending on the resistivity of the molten material. The production of heat maintains the working substance at its melting point. Since some heat is also gene-rated in the concentrator 41, the induction coil 43 is water-cooled. It was found desirable to maintain the concentrator 41 below the temperature of boiling water.

The radio-frequency energy reflector 40 is similar in shape to the concentrator 41, except that it has no slot 42 therein. The reflector 40 acts as a Faraday shield to protect the grown crystal 46 from the heating effects of the induction coil 43. The reflector 40 is also cooled by a coil 45 connected to the same water cooling system as used for the cooling of induction coil 43. In the actual equipment the aperture edge 48 of the concentrator 41 and the aperture edge 50 of the reflector 40 are electrically grounded.

Although only a few embodiments and methods incorporating the essence of the present invention have been specifically described herein, it should be clear that the methods are not limited thereto, in Particular, the shape of the concentrator coil, the direction of pulling the seed crystal, the geometrical configuration of the single crystal desired to be grown and the material forming the melt may be chosen to meet any desired design requirements.

What is claimed is:

1. Apparatus for growing a flat single crystal from a melt comprising: a crucible; means for drawing the growing crystal progressively upward from the melt; concentrator means supported at approximately the top of said crucible, said concentrator means comprising a conoidal shaped concentrator element of thin electrically conductive material supported with its major axis coaxial with that of said drawing means, said concentrator element havingat its apex a narrow elongated opening, said apex projecting downwardly toward said crucible, said concentrator element having a slot that extends radially outward from its apex to its base such that said concentrator element forms a single t-urn slotted conductor; a multipleturn coil surrounding and in physical contact with said concentrator element, said multiple-turn coil having one turn electrically connected to said concentrator element and the other turns electrically insulated therefrom in such a manner that said multiple-turn coil and said concentrator element form an autotransformer for increasing the electromagnetic current in said concentrator element over the current supplied to said coil; a conoidal shaped electromagnetic shield supported within said concentrator element, said shield having a narrow elongated aperture at its apex and terminating slightly above the apex of said concentrator element, said shield aperture being elongated in the same direction as the opening of said concentrator element so that it closely surrounds said grown crystal; and means supported by said shield for cooling said shield and said concentrator element.

2. The apparatus of claim 1 wherein the edges of said concentrator element and said shield are tapered to thin edges.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Hannay: Semiconductors, page 118 (1959), Monogram Series 140, Reinhold Publ. Co.

Printed application S38055VI/40d, Germany, June 14, 1956. 

1. APPARATUS FOR GROWING A FLAT SINGLE CRYSTAL FROM A MELT COMPRISING: A CRUCIBLE; MEANS FOR DRAWING THE GROWING CRYSTAL PROGRESSIVELY UPWARD FROM THE MELT; CONCENTRATOR MEANS SUPPORTEED AT APPROXIMATELY THE TOP OF SAID CRUCIBLE, SAID CONCENTRATOR MEANS COMPRISING A CONOIDAL SHAPED CONCENTRATOR ELEMENT OF THIN ELECTRICALLY CONDUCTIVE MATERIAL SUPPORTED WITH ITS MAJOR AXIS COAXIAL WITH THAT OF SAID DRAWING MEANS, SAID CONCENTRATOR ELEMENT HAVING AT ITS APEX A NARROW ELONGATED OPENING, SAID APEX PROJECTING DOWNWARDLY TOWARD SAID CRUCIBLE, SAID CONCENTRATOR ELEMENT HAVING A SLOT THAT EXTENDS RADIALLY OUTWARD FROM ITS APEX TO ITS BASE SUCH THAT SAID CONCENTRATOR ELEMENT FORMS A SINGLE TURN SLOTTED CONDUCTOR; A MULTIPLETURN COIL SURROUNDING AND IN PHYSICAL CONTACT WITH SAID CONCENTRATOR ELEMENT, AID MULTIPLE-TURN COIL HAVING ONE TURN ELECTRICALLY CONNECTED TO SAID CONCENTRATOR ELEMENT AND THE TURNS ELECTRICALLY INSULATED THEREFROM IN SUCH A MANNER THAT SAID MULTIPLE-TURN COIL AND SAID CONCENTRATOR ELEMENT FORM AN AUTOTRANSFORMER FOR INCREASING THE ELECTROMAGNETIC CURRENT IN SAID CONCENTRATOR ELEMENT OVER THE CURRENT SUPPLIED TO SAID COIL; A CONOIDAL SHAPED ELECTROMAGNETIC SHIELD SUPPORTED WITHIN SAID CONCENTRATOR ELEMENT, SAID SHIELD HAVING A NARROW ELONGATED APERTURE AT ITS APEX AND TERMINATING SLIGHTLY ABOVE THE APEX OF SAID CONCENTRATOR ELEMENT, SAID SHIELD APERTURE BEING ELONGATED IN THE SAME DIRECTION AS THE OPENING OF SAID CONCENTRATOR ELEMENT SO THAT IT CLOSELY SURROUNDS SAID GROWN CRYSTAL; AND MEANS SUPPORTED BY SAID SHIELD FOR COOLING SAID SHIELD AND SAID CONCENTRATOR ELEMENT. 